The present invention relates generally to optical fiber communications, and particularly to an apparatus for the inducing birefringence to an optical fiber in a controlled manner.
Optical transmission systems, including optical fiber communication systems, have become a useful vehicle for carrying voice and data at high speeds. However, in an optical transmission system, waveform degradation due to polarization mode dispersion (PMD) in the optical transmission medium can be problematic, particularly as transmission speeds are ever-increasing.
For purposes of illustration, a single-mode optical fiber having an optical wave of arbitrary polarization may be represented as a linear superposition of two orthogonally polarized optical modes (e.g. orthogonally polarized HE11 modes). In an ideal optical fiber waveguide, the two optical modes are degenerate in terms of their propagation properties owing to the cylindrical symmetry of the waveguide. This degeneracy, which is the source of the term single mode, is realized to a greater or lesser degree in deployed fibers, depending on the manufacturing process of the fiber and the extent to which external mechanical forces act on the fiber in a deployed system.
Real optical fibers, therefore, contain some degree of anisotropy due to the unintentional loss of circular symmetry. Whether this asymmetry occurs during manufacturing or is due to external forces, the loss of circular symmetry gives rise to two distinct polarization modes, with distinct phase and group velocities. As such, polarization effects in single-mode fibers are a direct consequence of the unintentional loss of degeneracy for the polarization modes.
Accordingly, when subject to stress either due to fabrication, or external factors, a single-mode optical fiber now exhibits a loss of degeneracy of the two optical modes. This may be quantified as a difference in the local propagation constants for the modes:                                           β            s                    -                      β            f                          =                                                            ω                ⁢                                  xe2x80x83                                ⁢                                  n                  s                                            c                        -                                          ω                ⁢                                  xe2x80x83                                ⁢                                  n                  f                                            c                                =                                    ω              ⁢                              xe2x80x83                            ⁢              Δ              ⁢                              xe2x80x83                            ⁢                              n                eff                                      c                                              (        1        )            
where xcex2s is the propagation constant of the mode along the slow axis; xcex4f is the propagation constant of the mode along the fast axis; xcfx89 is the angular frequency of the light, c is the speed of light in vacuum; ns and nf are the effective indices of refraction for the slow and fast modes, respectively; and xcex94neff is the differential index of refraction, which is also referred to as the birefringence.
While the differential index of refraction, xcex94neff is typically approximately two to four orders of magnitude smaller than the respective indices of refraction for the fast and slow axes, as transmission speeds and distances are ever increasing, the affect of polarization mode dispersion may be become particularly problematic. To this end, the differential phase velocity which results from the difference in the local propagation constants given by equation (1), often results in a difference in the local group velocities for the two polarization modes. This differential group velocity may be particularly problematic in digital optical communication systems, where the optical signal is ideally a square wave.
In the time domain, the differential group velocity is manifest as a propagation time difference known as the differential group delay (DGD). The differential group delay may result in bit-spreading of the optical signals. Accordingly, the affect of this type of dispersion is a spreading of the original pulse in time, resulting in an overflow into a time slot of the transmitted signal which has been allotted to another bit. When the overflow becomes excessive, inter-symbol interference (ISI) may occur. ISI may result in an increase in the bit-error rate to unacceptable levels.
A variety of techniques for compensating for polarization mode dispersion are known. Generally, these techniques involve changing the birefringence of the fiber. It is known that a change in birefringence, as a function of applied stress, is typically greatest when the stress is applied transversely to either the fast axis or the slow axis of the optical fiber. Additionally, piezoelectric actuators are useful as fiber-squeezing devices as they are generally faster, use less power, and introduce less noise than electromagnetic based fiber squeezers. Finally, to dynamically compensate for polarization mode dispersion in a deployed system, it is often necessary to have a number of stress actuators (fiber squeezers) oriented at a predetermined angle relative to one another.
While the above-captioned conventional fiber-actuators for introducing compensating birefringence to an optical fiber in a deployed system have showed some promise, there are clearly drawbacks and shortcomings thereto. For example, many conventional actuators introduce axial strain. This strain (pulling) on the fiber may result in breakage and coating delamination of the fiber. Accordingly, this is not a robust technique. Moreover, conventional stress-actuators, which introduce stress either longitudinally or axially have proven difficult to align initially, and to maintain in alignment in a deployed system.
What is needed, therefore, is a stress actuator for compensating polarization mode dispersion which overcomes the drawbacks and shortcomings of the conventional devices described above.
According to an exemplary embodiment of the present invention, an apparatus for selectively introducing birefringence in an optical fiber includes an actuator which selectively exerts a force on the fiber, and a registration key which selectively orients an axis of the optical fiber at predetermined azimuths.
According to another illustrative embodiment of the present invention, an apparatus for changing the polarization state of an optical signal includes a plurality of sequentially connected phase shifters, wherein each of the phase shifters is adapted to exert a force on an optical fiber disposed therein. Each of the plurality of phase shifters includes a registration key which selectively orients an axis of the optical fiber disposed in the registration key at a predetermined azimuth.